APPARATUS FOR HANDLING PROCESS FLUID

Abstract: Apparatus for handling process fluid, that is a meter (10) includes a mass flow meter block (16). A deformable metal seal (114) is positioned between the mass flow meter block (16) and a thermal mass flow sensor (62) in deformable engagement with a pair of edges (37a, 39a) formed thereon to provide a low contamination, high integrity metal seal (114).

Description: APPARATUS FOR HANDLING PROCESS FLUID BACKGROUND OF THE INVENTION The invention relates in general to apparatus for handling process such as a flow meter having an improved metal seal between two of its components. In particular, the invention relates to a thermal mass flow meter having a metal seal which allows excellent dimen¬ sional control over the assembled dimensions of the mass flow meter while maintaining good sealing and low con- tamination of a gas stream flowing through the mass flow meter. It is well known in the art of designing and manufacturing mass flow meters and mass flow con¬ trollers, in particular mass flow controllers having thermal mass flow meters connected with electromagnetic valves, to assemble the components of the mass flow controller, including inlets, mass flow meter blocks valve blocks, outlets, and the like, with elastomeric or other nonmetallic O-ring fittings forming seals there- between. Such thermal mass flow controllers are often used to meter precisely the flow of a gas or vapor in the manufacture of semiconductor devices. For instance, semiconductor wafers are fabricated by a number of processes including chemical vapor deposition, thermal oxidation, plasma etching, and vacuum sputtering. Gases and/or vapors used in such wafer fabrication processes include hydrogen, oxygen, silane, dichlorosilane, sulphur hexafluoride, tetraethoxysilane, argon and nitrogen. It is also well known that the design rules for integrated circuits continue to shrink and are now approaching one-quarter micron. As the dimensions of the individual components formed on the semiconductor dies shrink, the likelihood increases that even very small amounts of particulates may so contaminate the process gas stream as to render the semiconductor dies unusable. In an earlier effort to reduce the number of particulates that may be generated by mass flow con- trollers having conventional O-ring seals, the inventors have previously developed a modular thermal mass flow controller having bead type metal seals that is dis¬ closed in International Publication No. WO 90/12239 (International Application No. PCT/US90/02019) . That modular thermal mass flow controller has separate mass flow meter and valve blocks that terminate in face seals. The blocks are held together by screws. Separate inlet and outlet fittings are attached to the face seals, the fittings being adapted to form a first portion of a Cajon fitting. The rounded metal or bead type seal formed integrally with flat faces not only allows a modular mass flow controller to be constructed easily from a variety of.components, but also avoids having to use intermediate Cajon fittings that would increase the size or footprint of the mass flow con¬ troller. Unfortunately, the prior bead type metal seal suffers from the drawback that it requires a relatively large force to bring it into good sealing engagement because the substantially circular cross section of the bead type seal has high crush strength. Since the bead is being engaged by flat faces of the mass flow con¬ troller component blocks, the assembly often may be uncertain how much the bead must be compressed in order to obtain good sealing between the blocks. What is needed is an improved metal seal which allows easy closure with a low applied force and assembly while providing excellent sealing characteris¬ tics and little or no particulate contamination of the process gas stream flowing through the thermal mass flow controller. SUM ARY OF THE INVENTION The present invention is embodied in an apparatus for handling process fluid, more specifically, a modular thermal mass flow controller incorporating low closing force, low contamination metal seals between components thereof. The apparatus includes a block having a sealing groove formed on a face thereof, the sealing groove terminating in an edge. A removable connection fitting is connected to the sealing groove in fluid communication therewith. The connection fitting has a sealing groove formed on a face. The sealing groove extends around an axis and is defined by a radial distance slightly different than the radial distance of the groove on the block. The removable connection groove also terminates in an edge. A deformable metal seal is positioned between the removable connection and the block so that as the removable connection and the block are assembled, the faces are moved toward each other and offset edges engage the seal. The pressure applied by the edges forces the seal into a configura¬ tion -having a pair of relatively wide end portions and a relatively narrow middle portion. The instant invention provides cost savings by reduction of seal thicknesses from 0.060 inch to 0.031 inch, using sheet stock rather than bar stock for production of seals. Since the seal is chemically machined and not machined, it may be etched in multiple units and may also be heat treated, cleaned and packaged in multiple units. A keeper assembly associated with the seal provides a self-locating feature so that the seal may be quickly and easily assembled. The high pressure applied by the edges upon the deformable metal seal yields a 60% reduction in the tightening torque required to effect adequate closure of the components allowing the apparatus to be assembled without the use of a clamping fixture. Furthermore, the wide range of deformability of the seal, while maintaining good con¬ tact, allows for large tolerances in the mating surface finishes of the seal, the edges and the associated surfaces of the block and removable connecting being brought into contact with each other. The likelihood of internal seal wall breakdown of the type which may occur in a metal O-ring design and which yields slow contam¬ inating out-gassing of the internal O-ring volume into the process stream is greatly reduced. Although the seal itself may be disposed of when the components are disassembled for maintenance and the like, the rela¬ tively hard surfaces of the block and removable connection are not damaged by repeated closures with the soft seals and high sealing integrity is thereby preserved. It is a principal aspect of the present inven¬ tion to provide an apparatus for handling process fluid, including an improved metal seal which provides very little in the way of contaminants to the process gas being handled. It is another aspect of the present invention to provide apparatus for handling process fluid which is low cost and quickly and easily assembled. Other aspects of the invention will become obvious to one or ordinary skill in the art upon a perusal of the following specification and claims in light of the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a normal flow thermal mass flow controller having a thermal mass flow meter embodying the present invention; FIG. 2 is a sectional view of the thermal mass flow controller shown in FIG. 1 taken along line 2--2 of FIG. 1; FIG. 3 is an elevational view of a first metal integrated keeper-seal embodying the present invention and adapted to be positioned between a thermal mass flow meter block and a valve block of the thermal mass flow controller shown in FIG. 1; FIG. 4 is an elevational view of a second metal integrated keeper-seal embodying the present invention and adapted to be positioned between a sensor tube mounting block and the thermal mass flow meter block of the thermal mass flow controller shown in FIG. 1; FIG. 5 is an elevational view of a third metal integrated keeper-seal embodying the present invention and adapted to be positioned between the valve block and an electromagnetic valve of the thermal mass flow con¬ troller shown in FIG. 1; FIG. 6 is an enlarged sectional view, having portions broken away to show details thereof, of a part of the first integrated metal keeper-seal shown in FIG. 3 prior to engagement by the thermal mass flow meter block and the valve block; and FIG. 7 is an enlarged sectional view, having portions broken away to show details thereof, of the first metal seal shown in FIG. 6 after it has been engaged by the thermal mass flow meter block and the valve block and forming a good seal therebetween. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, and especially to FIG. 1, apparatus for handling process fluid com- prising a modular thermal mass flow controller embodying the present invention and generally identified by refer¬ ence numeral 10 is shown therein. The modular thermal mass flow controller 10 includes an inlet 12 for receiving a flow of gas to be metered. A thermal mass flow meter 14 which is also a removable connection fitting for producing a signal related to a mass flow rate of a gas or vapor, for instance a process gas such as hydrogen, oxygen, silane, dichlorosilane, sulphur hexafluoride, tetraethoxysilane, argon and nitrogen, is connected to the inlet 12 to receive the flow of gas from it. The thermal mass flow meter 14 has a thermal mass flow meter block 16 in communication with the inlet 12. A valve assembly 18 is connected to the thermal mass flow meter 14 at the thermal mass flow block 16 to control the rate of flow of gas in response to the thermal mass flow meter 14. The inlet 12 includes an inlet block 20 having an integrally formed threaded fitting 22 of the type adapted to connect to a Cajon or VCR fitting to receive gas from a suitable source such as a tank or a portion of a gas shelf. The inlet block 20 is connected to the thermal mass flow meter 14 via a pair of bolts, one of which is shown as the bol.t 24. A cover 26 has a cover tab 28 that receives a screw 30 for holding the cover 26 to the inlet block 20. The inlet block 20 has a circu¬ lar gas inlet port 32 to which is connected a gas inlet bore 34 formed therein that has a substantially circular cross section and receives the process gas stream to be metered. That process gas stream exits the inlet block 20 through a circular face exit port 35 formed in a flat face 36. A rectangular cross section sealing groove 37 having a right angle sealing edge 37a is formed around the port 35. A circular mass flow meter inlet port 38 is positioned in registration with the exit port 36 to receive the stream of gas. A rectangular cross section sealing groove 39 having a sealing edge 39a is formed on a face 39b. The rectangular cross section sealing groove 39 has a slightly smaller diameter than the seal- ing groove 37. A bypass bore 40 formed in the thermal mass flow meter block 16 is connected to the mass flow meter inlet port 38 to receive and carry the stream of gas. A pressure dropping flow restricter or bypass 42, consisting of a plurality of tubes 44, rests within the bypass bore 40 to provide a pressure drop across the thermal mass flow meter block 14 and to drive a sensor portion of the stream of gas through a sensor tube of the thermal mass flow meter 14. The thermal mass flow meter 14 includes a signal processing circuit 46 for energizing thermally responsive windings in contact with the sensor tube and producing the flow signal. A thermal mass flow sensor 62 for generating a first unconditioned flow signal is connected to the thermal mass flow meter block 16 to receive a portion of the flow of gas therefrom. The thermal mass flow sensor 62 includes a sensor base 64 connected to the block 16 to receive the flow of gas therefrom. The sensor tube 68 is in this embodiment a 316L stainless steel sensor tube 68 that is of conventional construction in the thermal mass flow meter art. The sensor tube 68 receives a portion of the flow of gas from the thermal mass flow meter block 16. The sensor tube 68 has an inlet leg 70 connected to the sensor inlet bore 71 of the mass flow meter block 16 to receive a sensor portion of the stream of gas therefrom, a transverse or sensor leg 72 is formed integrally with the inlet leg 70 and receives the sensor portion of the stream of gas from the inlet leg 70. An outlet leg 74 is formed integrally with the transverse leg 72 and receives the sensor stream therefrom. The sensor portion of the flow of gas is forced through the sensor tube 68 by the pressure drop across the bypass 42. The circuit board 48 is mounted behind the sensor tube 68. A pair of serially connected 1.5 mil electrical windings 76 and 78 having a high coefficient of thermal resistivity is wound around the transverse leg 72 of the sensor tube 68 in good heat transfer relation as is well known to those skilled in the art. A thermal strap 80, in this embodiment comprising a copper wire, is connected at a first connection point comprising an upstream winding 82 to the inlet leg 70 by soldering or silver epoxy or other good thermal con¬ ductor. An opposite end of the thermal strap 80 comprises a second point 84 that is connected in good heat transfer with the outlet leg 74 of the sensor tube 68. The thermal strap 80 both balances the thermal response and speeds up the thermal response of the sensor tube 68. An insulating blanket 86 is wrapped around the transverse leg 72 and the serially connected windings 76 and 78 to minimize the effect on the sensor response to the flow of gas of unwanted heat transfer out of the transverse leg 72. When the windings 76 and 78 are electrically energized by electric current from the signal processing circuit 46 they provide a flow signal through a wiring harness 89 to the signal processing circuit 46 connected thereto. The flow signal is received by the signal processing circuit board 46 and transmitted to and from an edge connector 88 of the circuit board 48 extending through the top of the cover 26. The valve assembly 18 has an electromagnetic valve 92 connected to the circuit board 46 to be driven therefrom in response to the flow signal. In particu- lar, the electromagnetic valve 92 includes a solenoid 94 that drives a plunger 96 toward and away from a jet 98 to control the flow of gas or vapor through the thermal mass flow controller 10. A valve block 100 is likewise connected by a pair of threaded connectors, one of which is shown as connector 101, to the mass flow meter block 16. Both threaded connectors 30 and 46 are adapted to receive an Allen wrench in order that they may be tightened into engagement with the mass flow meter block 16. A valve inlet bore 102 that receives the flow of gas from the mass flow meter block 16 and carries it to an inlet 104 of the electromagnetic valve 92. An outlet 106 is connected to the valve jet 96 to receive the flow of gas from the electromagnetic valve 92 and transmitting it to a substantially circular outlet bore 108 extending through an integral outlet Cajon fitting 110 having a threaded outer wall 112 formed thereon. Metal seals are provided respectively between the inlet block 20 and the mass flow meter block 16, between the mass flow meter block 16 and the valve block 100, between the mass flow meter block 16 and the sensor 62, and between the valve block 100 and the electromagnetic valve 96. As is shown in FIGS. 3, 4, and 5, which show integrated keeper-seals 114, 116 and 118 in the form they take prior to assembly, the inte¬ grated keeper-seal 114 is of the type which is posi- tioned between the pairs of the blocks 20, 16 and 100. The integrated keeper-seal 116 shown in FIG. 4 is of the type positioned between the mass flow meter block 16 and the sensor 62. The integrated keeper-seal 118 shown in FIG. 5 is of the type used to effect sealing joinder between the valve block 100 and the electromagnetic valve 92. The integrated keeper-seal 114 includes a keeper portion 120 and a rounded seal portion 122. The integrated keeper-seal 114 is formed by chemical etching with the keeper portion 120 being etched to a thickness of 0.026 inch and the rounded seal portion 122 being etched to a thickness of 0.031 inch. The rounded seal portion 122 shown in FIG. 6 has a rounded rectangular cross section and a pair of substantially parallel opposing faces 126 and 128. The integrated keeper-seals 114, 116 and 118, are comprised of fully annealed nickel 200 having a Rockwell hardness of 35 to 45 R B . No surface finish is required on the rounded seal portion 122 or on any other portion of the integrated keeper- seals 114, 116 and 118. When the inlet block 20 having the face 36 is brought into proximity with the mass flow meter block 16 having the face 39b, as shown in FIGS. 6 and 7, the rectangular cross section sealing groove 37 in the inlet block 20 is brought into offset proximity with the rectangular cross section sealing groove 39b of the mass flow meter block 16. Note that the rectangular cross section grooves 37a and 39b, however, are offset so that the sealing right angle edge 37a and the right angle edge 39a are aligned and brought into shearing engage¬ ment respectively with faces 126 and 128 of the rounded seal 122. The pressure exerted by the faces 126 and 128 upon the seal 110 compresses and deforms the rounded seal 122 into a dumbbell-shaped cross section having a first widened end 150, and a second widened end 152 and a narrow center portion 153. Only 60% of the closure force for bead type seals is required because the inventive seal is, in essence, two edges which are contacting the rounded seal 122. In addition, because of the very high local pressures applied by the edges 37a and 39a to the rounded seal 122, no surface finish is required to prepare the rounded seal 122 prior to closure, as would be needed for the bead-type seal of the prior art. Referring now to the integrated keeper-seal 116 shown in FIG. 4, the integrated keeper-seal 116 comprises a 0.026 inch thick keeper plate 170 to be engaged by the threaded fasteners during assembly so that a pair of sensor circular seals 172 and 174 is in proper registration. The sensor circular seals 172 and 174 are connected to the keeper plate 170 by respective webs 176, 178, and 180, as well as webs 184, 186, and 188. Each of the circular seals 172 and 174 is 0.031 inch thick and has a rounded rectangular cross section. Similarly, the integrated keeper-seal 118 shown in FIG. 5 includes seal rings 200, 202, 204 and 205 each having rounded rectangular cross sections and a thickness of 0.031 inch. Only one of the seal rings 200 or 202 is adapted to match with the inlet 56 of the electromagnetic valve 100. The other seal ring is a spare to allow a symmetric keeper to be used. The seal ring 204 provides a seal immediately underneath the valve jet 98. The seal ring 205 may also be seen in FIG. 2. The seals 172, 174, 200, 202, and 204 are likewise held between knife edges integral with the closing faces, causing a dumbbell-shaped, deformed seal to be formed therein with relatively low closing force without the introduction of contaminants or the need for surface finishing on the seals themselves. The seal rings 200 and 202 are connected to the seal ring 204 via respective webs 206 and 208. A web 210 connects the seal ring 200 to a plate 212 of the keeper assembly 104. A web 214 connects the seal ring 202 to the keeper assembly 212. It may be appreciated that the webs may shear from the seal rings, as is best seen in FIG. 7, as the seal is being completed when the modular thermal mass flow controller 10 is assembled. Such web shearing, however, does not result in any reduction of the integrity of the seal and whether the shearing takes place or not is immaterial to the high integrity of the seal thus being formed. It may be appreciated that the low cost, low contamination, easily produced mass flow controller having a metal seal may be used in very clean environ¬ ments for the production of semiconductors. While there have been illustrated and described particular embodiments of the present inven- tion, it will be appreciated that numerous changes and modifications will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention.

Claims: What Is Claimed Is: 1. Apparatus for handling process fluid, comprising: a block having a block sealing groove formed on a face thereof and terminating in an edge; a removable connection fitting connected to the block in fluid communication therewith, the remov¬ able connection fitting having a connection sealing groove formed on a face thereof, the sealing groove extending around an axis and being defined by a radial distance slightly different than the radial distance of the block sealing groove, said connection sealing groove terminating in an edge; and a metal seal positioned between the removable connection fitting and said block so that as said remov¬ able connection fitting and said block are assembled, said faces are moved toward each other and said offset edges engage said metal seal deforming said metal seal into a configuration having a pair of relatively wide end portions and a relatively narrow middle portion.

Inventors: VU KIM N, VU, KIM, N.
Applicants: UNIT INSTR INC, UNIT INSTRUMENTS, INC.
Application year: 1994
Main classification: F16L25
IPC codes: F16J15, F16L23, F16L25, F16L55, G01F1